Method of coring to preserve fluid content



Feb. 7,1939. N. JOHNSTON 2,146,263

METHOD OF CORING TO PRESERVE FLUID CONTENT Filed May 31, 1958 NORRISJOHNSTON INVENTOA QM w. g y

Patented Feb. 7, 1939 UNITED STATES METHOD OF CORING T PRESERVE FLUIDCONTENT Norris Johnston,

Los Angeles, Calif.

Application May 31, 1938, Serial No. 210,842

Claims.

The invention pertains to the taking of cores from the formations at thebottoms of deep wells such as oil wells, and more specifically tomethods of cutting such cores without allowing the intrusion of anydrilling fluid into them and to methods for bringing such cores to thesurface of the earth without loss of any part of their original contentof gases or liquids.

In modern deep well drilling practice, coring ahead is resorted to inorder to determine in advance the lithologic character of the structureto be penetrated by the drill. This practice becomes increasinglynecessary and desirable as horizons suspected to be capable of producingoil or gas are approached.

The methods of coring heretofore used are adequate for obtainingaccurate samples of the rocks from which the cores are taken, at leastin most cases, but they are wholly inadequate to cut and bring to thesurface a core having its original fluid contents intact anduncontaminated, This follows from the fact that all present coringmethods expose at least one surface of the core to contact with thefluids in the well during the cutting of the core and its withdrawal.

Since in most cases it is found expedient to keep the hydrostaticpressure of the drilling fluid in the wall substantially higher than thefluid pressure in the formation, to avoid any possibility of theformation fluids flowing out of the hole and thus interfering withdrilling, it is normally true that the drilling fluid, in direct contactwith the core throughout the period of cutting, intrudes into the corethroughout the period of cutting, in trudes into the core under adefinite and substantial pressure differential, thus displacing connatefluids in an uncontrollable manner and to an unknown extent.

One of the purposes of the present invention is to remedy thisinadequacy by providing that the core shall be out without contact atany time with the drilling fluid, and under such conditions as to befree from the influence of the excess pressure of that fluid over theformation fluid pressure. In general terms, the means for attaining thisresult is to core with a barrel of special design, through a smallamount of molten metal at the bottom of the hole. This layer of metal,being about seven to ten times as heavy as most drilling fluids, will.completely displace them from the bottom of thehole 'where coring is inprogress, thus effectively preventing core contamination. The metalitself will not intrude into the core as it does not wet the materialsof the formation and its high surface tension prevents intrusion intoeven fairly porous core bodies, especially if the drilling fluidpressure is not allowed to exceed formation pressure by too wide amargin.

A further purpose of the invention is to bring the core to the surfacewith its original fluid contact intact. As the withdrawal of the core iseffected, under present methods, with at least part of the surface ofthe core exposed to contract with drilling fluid, and under a constantlylessening hydrostatic pressure as the surface of the earth isapproached, any gases in the core, which originally are highlycompressed, are allowed to expand and escape, carrying with them thewater or oil which may be present in the formation from which the corewas taken. Thus, by the time the core reaches the surface, its originalcharacter as regards fluid contents is totally obscured, andascertainment of the quantities and kinds of fluid originally containedis rendered impossible.

This loss of gas and other fluids I prevent by controlling thetemperature of the mud circulation, thus freezing the metal into a solidplug at the bottom of the core holder. This step allows the sample to bebrought to the surface-in a hermetically sealed container, the contentsof which can be studied at leisure in the laboratory and from which nofluid can escape until it is opened deliberately.

While various types of coring apparatus may be used in the practice ofthe invention, I shall illustrate its general application with referenceto the. particular form shown in the attached drawing and hereinafterdescribed.

Referring to the figure, 1 is a drilling bit, preferably of suchdiameter as to form an undersized hole 2 at the bottom of the main hole3. A core barrel centering ring is indicated at 4 and the core barrel at5, the lower end of this barrel being formed as a coring bit 6 and beingprovided with the usual core catcher I and also with grooves 8, thepurpose of which will appear. This barrel differs from the conventionalin being open at the lower end only, the upper end being blind asindicated at 9. The entire inner surface of the barrel, and particularlyits lower portion, should be tinned to permit it to be wetted by thefused metal hereinafter described and thus to form a gas-tight jointwith the plug of metal after its solidification, As the barrel, afterthe solidification of this plug, forms a sealed bomb, it must be of suchstrength as to withstand internal pressures at least equal to themaximum pressure existing in the formation from which the core is to betaken.

Having provided the above simple apparatus, I

proceed in the following general manner to the taking of the desiredcore. I first obtain, by any suitable method, the temperature of thecirculating mud at the bottom of the hole during normal drilling andalso the static bottom temperature of the hole. I then drill anundersize hole one to two feet in depth as illustratedthis step is notessential but conserves the expensive alloy metal.

About two hours, or a sufficient time, before the core is to be taken, Istart the circulation of hot mud. By hot mud I mean circulating mudheated to a temperature above the normal temperature of the circulationin the particular well and also above the melting point of the alloymetal to be used. The mud may be heated in any convenient manner, as bythe introduction of steam into the circulation. When the well has becomeheated above its normal temperature, I set the kelly aside and drop intothe well a slug (as for example, more or less thirty pounds) of thefusible alloy, which melts on contact with the hot mud and sinks to thebottom of the well to form a pool on its bottom.

I then drill and core a short distance farther, as for example sixinches, to get into formation which has not been touched by drillingmud, the

alloy remaining on the bottom of the hole. I

then remove the center bit and leave the melted alloy on the bottomprotecting the newly exposed formation from contact with the drillingmud, which it does efiectively because of its high spe- ClfiC gravity.

As the next step I introduce the special form of core barrel abovedescribed and core down for a desired distance, as for example 12 to 18inches, through the melted alloy. The dotted line Ill-l of the drawingindicates the position and contour of the bottom of the hole before themetal is introduced.

Circulation of cold mud is then started and is continued until themolten alloy, which now surrounds the core and also the lower portion ofthe core barrel, has solidified. In the use of the term cold mud I referto circulating mud cooled below the normal temperature of the well andbe low the solidifying point of the alloy used. The freezing of thealloy should require but a relatively short time, as for example 45minutes. It is desirable at this point to raise the drill a few inchesfrom the bottom of the hole and maintain the cold circulation for one ortwo hours to make sure of solidification of the metal.

The drill stem is now withdrawn and the special core barrel broken out.If the general course of the above procedure'has been followed, theportion of the barrel below the core will be found to be filled andhermetically sealed with solidified metal, the grooves 8 preventing theinternal pressure from dislodging the metal plug. The core above thisplug will not have been subjected to any lowering of pressure nor tocontact with any foreign fluid, and will contain whatever kind andquantity of oil, gas, and water which it contained when in place in theformation.

The entire barrel may now be placed in a sealed bomb, the airsurrounding the barrel swept out with some suitable gas such as carbondioxide, the bomb heated to such temperature as to fuse the metal plugand cause the molten metal to drain away, and the evolved gases ventedfrom the bomb through measuring and analytical apparatus suited to thedeterminations of kind and quantity which are to be made. The bomb maythen be opened for the removal of the core at atmospheric pressure andfor the determination of any quantities of water or oil not removed bythe escaping gases.

The choice of alloy melting point rests on the temperatures existing inthe particular well from which the core is to be taken. For example, ifthe static or formation temperature is F. and the normal temperature ofthe circulatin fluid is 150 F., a suitable melting point for the alloywould be 155 F. The metal would then be melted with some rapidity byraising the mud fluid temperature to 175 F. and would be solidifiedwithin reasonable time limits by lowering the temperature to 135 F.Alloys ranging in melting point, in steps, from 0 F. to F. would coverthe entire range of wells, so far as I am aware, and could be providedin advance or made up to suit each specific case.

Alloys having definite melting points and sufficient mechanical strengthto withstand the internal pressures realized on withdrawing the corebarrel from even very deep wells may be made from suitable combinationsof bismuth, lead, tin. cadmium, and antimony. The preparation of suchalloys at desired melting points is a well known art, and it issufficient to say that the proportions of these five constituents, inthe order stated, would range from 50:27:13;10;0 to 50: 5:12:8:5, thefirst named having the minimum and the last the maximum melting point.

The density of these alloys is high, of the order of 600 pounds percubic foot; so that core cuttings will float up to the region of mudcirculation and the mud jets will not readily disturb the upper surfaceof the pool of molten metal. Penetration of the mud downwardly, throughor around the metallic pool, is obviously impossible. Loss of alloy byleakage into the formation is minimized by the high surface tension ofthe liquid metal, but care should be exercised to avoid excessivepressure difference between the mud fluid and the formation at' thebottom of the hole. On the withdrawal of the core barrel the portion ofthe metallic pool outside it may remain in the hole as a metallic ringor lining. This may readily be drilled away or may be recovered with aspear-head fishing tool if of sufficient value to repay the cost ofremoving it.

In order to prevent the intrusion of mud into the core barrel whilelowering it into the hole it may be desirable to fill the core barrelwith the low melting point alloy. When solidified this will retain itsposition until the temperature is raised to its melting point. If thefluids in the hole should be of such temperature that the metal willmelt while the barrel is being lowered to position, the filling may beretained by'a thin cap or membrane of some higher melting point metalplaced over the end of the barrel. This membrane should be of suchslight thickness as to be immediately destroyed when the core barrel isrevolved on the bottom of the hole.

I claim as my invention:

1. The method of coring the bottom of a drilled well which comprises:elevating the temperature of the circulating fluid in .said well aboveits normal temperature when drilling; introducing into said well a metalfusible at said elevated temperature; forming a core at the bottom ofthe hole and below the pool of molten metal collected thereon and withina core barrel open at only its lower end; lowering the temperature ofsaid circulating fluid below the solidifying point of said molten metaland thereby sealing said core within said barrel, and withdrawing saidbarrel with said core sealed therein.

2. The method of coring the bottom of a drilled well which comprises:forming a pool of molten metal on the bottom of said well; forming acore from said bottom and within a core barrel open at only its lowerend; detaching the core from said well bottom, and sealing the openlower end of said core barrel below said core by solidifying said moltenmetal before said barrel is withdrawn from said well.

3. A method of taking a core from the bottom of a drilled well whichincludes the step of solidiing the core within an otherwise closed corebarrel by solidifying a fused metal Within the open end of said barrel.

5. A method substantially as in claim 2, including the additional stepof forming a hole of reduced diameter in the bottom of said well with-10 in which said molten metal may collect.

NORRIS JOHNSTON.

